Diesel oxidation catalyst (DOC) systems, diesel particulate filter (DPF) systems, NOx occlusion catalyst (lean NOx trap (LNT) or NOx storage reduction (NSR)) systems, urea selective catalytic reduction (SCR) systems, and so on are in practical use for exhaust gas aftertreatment devices of diesel engines.
The DOC system and the DPF system are effective systems for reducing PM. Although the DOC, which is provided at an upstream position in the exhaust passage, is not capable of oxidizing solid soot, the DOC oxidizes a large portion of soluble organic fraction (SOF), which accounts for 30 to 70% of the total PM, and also removes hydrocarbon (HC) and CO at the same time. The DPF, which is provided at a downstream position, is formed of porous ceramics or the like having a fine pore size and captures a large portion of the PM contained in the exhaust gas.
A NOx occlusion reduction catalyst is constituted by a catalyst carrier of alumina (Al2O3) or the like, with a noble metal catalyst (e.g., Pt and Pd) and an occlusion material having a NOx occluding property (e.g., alkali metal including Na, K, and Cs, an alkaline-earth metal including Ca and Ba, and a rare earth including Y and La) supported on a surface of the catalyst carrier. The NOx occlusion reduction catalyst exhibits a function of either occluding NOx or discharging and purifying NOx depending on the oxygen concentration in the exhaust gas.
With a purification system having the NOx occlusion reduction catalyst, when the oxygen concentration in the exhaust gas is high (lean air-fuel ratio) as in a normal driving condition, NO in the exhaust gas is oxidized into NO2 by the noble metal catalyst or the like, such as Pt and Pd, and the occlusion material stores NO2 in the form of a nitrate (Ba(NO3)2) so as to purify NOx.
However, if the occlusion material continues to occlude (collect and retain) NOx, the occlusion material becomes saturated with the nitrate and loses its occlusion property. Thus, the driving condition is altered, and exhaust gas recirculation (EGR), post-injection of fuel, or exhaust pipe injection is carried out in a condition of low oxygen concentration to produce a rich state, and the fuel is reduced over the noble metal catalyst so as to produce CO, HC, and H2 in the exhaust gas. Thus, NOx is reduced, discharged, and purified.
In this manner, the purification system having the NOx occlusion reduction catalyst stores NOx when the air-fuel ratio is lean (when the oxygen concentration is high), and reduces and purifies the stored NOx when the air-fuel ratio is rich.
The NOx occlusion reduction catalyst adsorbs and stores SOx contained in the exhaust gas in addition to NOx. Unlike NOx, SOx cannot easily be desorbed. In order to release S from the occlusion material, the ambient temperature of the catalyst is raised to a high temperature (no less than 650 degrees C.) and the air-fuel ratio is controlled to be rich. This changes Ba2SO4 to carbonate+SO2, and desulfurization is achieved. Therefore, the NOx occlusion reduction catalyst needs to be regenerated by carrying out desulfurization control (S purge) at predetermined driving-distance intervals. During the S purge, an amount of EGR is set greater than that in a normal state and the air-fuel ratio is brought close to the stoichiometric ratio. Thus, an amount of emitted NOx becomes smaller due to a reduction in the NOx concentration at the exit of the engine and a ternary function. Therefore, a problem of NOx does not worsen.
When a selective catalytic reduction (urea SCR) unit is used, an amount of emitted NOx at the exit of the engine is reduced by reducing an amount of air intake during the PM regeneration. Therefore, a problem of NOx does not worsen.